Australian black coals are generally low in sulfur and trace elements but have a high ash yield which is usually a refractory ash, that is, it has a high ash fusion temperature, compared with overseas coals. This refractory ash reflects the high silica and kaolin clay content in the coal.
There are however some coals in Australia and many coals originating from overseas that have appreciable contents of other minerals which are more reactive and detract from the quality of the coal and/or hinder its industrial application.
In conventional coal washeries, as described in "An Introduction To Coal Preparation" edited by members of the executive committee of the Coal Preparation Society of NSW (1985), the main objective is to lower the ash yield of the coal without appreciable size reduction. The coal size generally ranges from about 150 mm to below 0.5 mm and depending on the size range, different washing techniques are used to separate the minerals and high ash coal from the coal rich fraction.
It is to be noted that the majority of techniques are based on the separation by density differences between coal at 1.3SG and minerals at 2.5SG. However, none of these methods are intended to alter the relative proportion of the individual minerals present in the coal. Thus any selective separation on size reduction or beneficiation at the washery is usually incidental.
It is known in the art that there are a variety of specialized, sophisticated chemical leaching techniques to demineralize coal wherein all of the minerals are removed to produce ultra-clean coal, that is coals having an ash of less than 1%. These methods are generally used to remove all minerals unselectively. Typically such methods employ hydrofluoric acid or fluoride salts as described by Lloyd, R. and Turner, M. J. (Kinneret Enterprises Ltd.) Patents pending and by Das, S. K. "Electrode grade carbon from coal by acid leaching process", Light Metals, 575 (1979); aqueous caustic soda solutions as described by Meyers, R. A, Hart, W. D. and McClanathan, L. C. "Gravimelt process for near complete chemical removal", Coal Processing Technology, 7, 89. (C.E.P. Technical Manual published by the American Institute of Chemical Engineers) (1981) and aqueous caustic soda solutions under autoclave conditions as described by the present inventors in "Removal of Mineral Matter from Bituminous Coals by Aqueous Chemical Leaching" Fuel Processing Technology, 9 217-233 (1984), "Demineralisation--A New Approach to Old Problems in the Utilisation of Solid Fuels", Proc. Aust. Inst. Energy Nat. Conf., Melbourne 347-357 (1985) and " An integrated, physical and chemical approach to coal beneficiation" Proc. CHEMECA 86, Adelaide, 297-302 (1986).
Additionally, selective chemical leaching has been used to remove pyrites from coal. The methods used are reviewed by Morrison, G. F. "Chemical desulphurisation of coal". Report No. ICTIS/TR15 I.E.A. Coal Research, London, June 1981. In this review the author categorizes the reactions possible as displacement reactions, acid/base neutralization, oxidation or reduction. It is noted that these methods are selective for pyrites only.
Another selective chemical leaching process is the ion exchange method used on lignites or brown coal to exchange calcium, magnesium, aluminium, iron and other cations from the carboxylate and phenate salts in the coal structure. These ion exchange processes are described by Bowling K. McG. and Rottendorf, H. R. in Austrailian patent specification 472,900, New Zealand patent specification 171,005 and Canadian patent specification 100,023 and in "Demineralised brown coal as an alternative to current hydrocarbon resources" Proc. 4th National Conf. Chem. Eng. Adelaide, 86-91 (1976). In this ion exchange method various solutions of salts and acids are used but there is no removal of minerals which are not chemically combined minerals, apart from quartz by fluoride salts.